Unlike initial task interest, very few researchers have investigated the manner in which sources of incentives, i.e., subject-determined versus experimenter-determined, influence the effects of incentives on intrinsic motivation. An experiment in intrinsic motivation was conducted to examine the effects of various sources of incentives and task attractiveness. College students (N=62) volunteered to participate in a purported marketing study involving the rank-ordering of the attractiveness of toys. Dependent measures were latency and duration of self-initiated play assessed during a period in which subjects were unaware of being observed and when incentives were no longer available. Results showed that subjects who received incentives, regardless of the source, initiated play earlier; therefore an enhancement effect rather than an undermining effect of incentives was found. Play was also initiated earlier by subjects in the high attractiveness condition than by those in the low attractiveness condition. The findings suggest that subjects' perceptions of success may mediate the effects of incentives on intrinsic motivation. (Author/JAC)